Sil - Mitsubishi Electric Melsec-QS Series Safety Application Manual

3
RISK ASSESSMENT AND SAFTY LEVEL

3.3 SIL

SIL is standardized in IEC61508.
The risk graph to be used for the SIL selection is shown in Figure3.4.
SIL selection starting point
In SIL, the following target failure measure is defined according to the level.
SIL
4
3
2
1
Degree of Exposure
influence frequency
Minor
Low
Moderate
High
Major
Low
High
Low
Enormous
High
Definition of symbols:
Symbol
1,2,3,4
Figure3.4 SIL risk graph
Table3.5 Target failure measure (PFD,PFH)
Low demand mode of operation
-5 -4
10 PFD 10
-4 -3
10 PFD 10
-3 -2
10 PFD 10
-2 -1
10 PFD 10
*1: For the low and high demand modes of operation, refer to IEC61508.
Probability of risk
Possibility of
event occurrence
risk event
Frequent
avoidance
a
Avoidable
1
Hard to avoid
2
Avoidable
Hard to avoid
3
Avoidable
Hard to avoid
4
Avoidable
Hard to avoid
b
-,a No safety requirements.
b Not sufficient with a single safety-related system.
Safety integrity level
Stands for SIL1, SIL2, SIL3 and SIL4 respectively.
*1 High demand mode of operation
-9
10 PFH 10
-8
10 PFH 10
-7
10 PFH 10
-6
10 PFH 10
Moderate Infrequent
a
1
a
2 1
3 2
4
3
Definition
(Referred to IEC61508-5.)
*1
-8
-7
-6
-5
(Referred to IEC61508-1.)
3
3.3 SIL
1
2
3
4
5
- 5